Progress in the production of concrete beams (also known as “girders”) for construction of bridges and buildings was greatly stimulated in the 1950's when the technique of prestressing the concrete was proven to have advantages in the United States. There are two known techniques of prestressing: pre-tensioning and post-tensioning. Both techniques of prestressing employ steel cables or bars to apply and hold a concrete member in compression. Prestressing can be referred to as “active reinforcement” as compared to “passive reinforcement”, such as is obtained with mild reinforcing steel (rebar).
Pre-tensioning is the predominate form of prestressing employed in the precast concrete industry. This technique involves stretching steel cables with a high tensile force, with the cables held between fixed abutments that are situated at each end of a casting platform, or “bed”, and placing concrete in forms on the bed, which forms encase the cables to form a beam or girder. At a later time, after the concrete has gained sufficient strength and bonded well to the cables, the cables are released from the abutment anchors, the forms removed, and the completed beam or girder is lifted from the bed.
It is of great economic importance that this procedure be completed on a daily cycle. In order to do this, heating the newly cast concrete at a curing temperature as high as 180 degrees F. to accelerate concrete strength gain has been a common practice.
The post-tensioning technique is not generally practiced in precast concrete production. Post-tensioning is more expensive per pound than pre-tensioning, so it has been employed sparingly in the production of beams or girders other than in special cases to meet design requirements.
In recent years, there has been remarkable progress in making concrete that has much higher strength than ever before. Ultimate 56 day compressive strength is now possible in the 10,000 psi to 20,000 psi range, which is up to 10,000 psi higher than strengths attainable a short time ago.
However, there has not been an advantage taken of higher strength concrete by employing a proportionately higher prestressing force in the design of precast beams or girders. Beam or girder load carrying capacity is increased dramatically when, using the same beam or girder size and shape as those made with “standard” concrete, stronger high performance concrete (HPC) is employed with a substantially greater prestressing force. This fact was demonstrated on an experimental bridge project where HPC beams having a 56 day strength of 13,600 psi were constructed with approximately 60 percent more prestressing force than standard beams made with 6,000 psi concrete. Test results proved that four HPC beams had the same load carrying capacity as seven standard beams for “twin” bridges of an identical span and roadway width. Although the cost per beam was higher for the HPC beams, the cost of the bridge superstructure having four high structural capacity beams was approximately 15% lower than the bridge having seven standard beams. This project confirmed the economic viability of employing higher structural capacity beams made with superior concrete strength and constructed with a high prestressing force. However, industry has not reaped the benefits of these features to achieve an improved and more economic product. There are certain problems that must be solved.
In addition to the common precautions observed in the design of a concrete mix, there are two important factors that must be dealt with concerning concrete durability. Both of these factors pose potential problems in making durable concrete beams or girders, as well as other concrete members. The first is known as alkaline-silica reaction (ASR); the second is called delayed ettringite formation (DEF). ASR is caused in large part by high alkalinity in the concrete reacting over time with silica in the aggregate. In severe cases, which are not uncommon, this reaction results in cracking and destruction of the concrete.
On the other hand, it has been learned recently that DEF is promoted principally by curing the concrete at a very high temperature. DEF typically occurs over time in mature concrete. It has been mistakenly identified as ASR in some cases, because its apparent failure mode is similar to the failure mode attributable to ASR.
The solutions to both problems are now known. Damage due to ASR can be avoided by substituting another cementitious material such as fly ash or slag for a portion of the cement in the mix to reduce net alkalinity. The drawback to this approach is that early concrete strength gain is slowed. Although final strength is typically very high, the concrete strength required for transfer of stress (the “release strength”), is not reached in time for daily recycling on the prestressing bed. Daily recycling of the bed is critical to a beam or girder manufacturer's economics.
DEF can be avoided by restricting concrete curing temperature to a maximum of approximately 160 degrees F. Here again, because early concrete strength gain is dependent on curing temperature, the lower temperature requirement makes attaining release strength overnight less likely.
Thus, there are two factors that have constrained production of superior and more cost-effective beams or girders prefabricated with HPC. Since higher strength concrete beams or girders containing a high prestressing force have been shown to produce a significant lower cost for a completed structure, it is important to have a way of making prestressed HPC beams or girders on a daily production cycle.
Control of camber in concrete beams or girders can be yet another serious problem. Camber is the arching upward of a beam/girder or slab that is prestressed when the prestressing force is located below the centroid of the concrete. In almost all cases, the pre-tensioning force applied to a beam or girder on a pre-tensioning bed is well below the centroid of the concrete. When a prestressing force (which is a compressive force) is applied to concrete, the concrete immediately shortens elastically as the force is applied. Thereafter, there is an inelastic shortening due to a phenomenon known as “creep” of the concrete. The amount of creep is a function of time, the level of compressive stress, and the modulus of elasticity of the concrete. Camber takes place in a prestressed concrete beam or girder when the concrete fibers in the lower portion of the member are under a higher compressive stress than the fibers in the upper portion. Creep of the concrete continues to shorten the bottom of the member as time passes, causing camber to grow. There have been cases where camber growth has been so great that beams or girders became unfit for use in structures and were rejected. The economic implications of such a problem go well beyond loss of money by the precaster having to manufacture substitute beams or girders. The construction company, depending upon timely delivery of product for constructing the bridge or building, is impacted by delay that ensues while new beams or girders are manufactured to replace the rejected ones.
One objective of the present invention is to provide a process that can be readily implemented by beam or girder manufacturers to overcome these problems.